1. Technical Field
The present disclosure relates to surface acoustic wave (SAW) sensors for detecting the growth of biofilms. More particularly, the present disclosure relates to piezoelectric SAW sensors having passivation film layer.
2. Background of Related Art
Bacteria can attach to surfaces and form microcolonies as their population increases. The colonies eventually can form a community known as a bacterial biofilm [1, 2]. A biofilm is not simply a group of bacteria, but a complex collection of microorganisms encased in an extracellular matrix. The extracellular matrix is composed of exopolysaccharide polymers which promote irreversible adhesion of microcolonies on the surface and also prevent diffusion of antibiotics through the biofilm [1, 2]. Due to the complex extracellular matrix and heterogeneous bacterial composition, biofilms are resistant to bacteriophages in industry and to chemically diverse antibiotic treatments in clinical fields [3]. In addition, bacterial corrosion of metals is an economically important consequence of bacterial biofilm formation that illustrates several fascinating aspects of the structure and physiology of these adherent bacterial populations. Therefore, environmental, clinical, and industrial long term reliable biofilm growth monitoring is critical to prevent contamination, severe infection, and corrosive problems due to the biofilm formation.
The measurement of bacterial biofilms with capacitive sensing has been applied by Yang and Li to monitor Salmonella typhimurium bacteria [4], and by Ghafar-Zadeh et al. to detect Escherichia coil (E. coli) [5]. In Yang and Li at al. [4], an interdigitated microelectrode was fabricated to provide detectable impedance signals in capacitance measurement during bacterial growth, S. typhimurium bacteria were grown over the microelectrode and the capacitance change was continuously measured. Capacitive sensing in a liquid environment, however, can be interfered with by a conductive media due to the current flow through the growth media [4].
The direct impedance measurement of the attachment of E. coli on an electrode is demonstrated by other groups [6, 7]. The change of impedance during bacterial growth is correlated with the biomass adhere on the electrode. This impedimetric sensing is particularly useful in detecting very early attachment of bacteria based on the significant impedance change observed upon attachment. However, long-term real-time biofilm monitoring by impedance measurement requires a continuous current source for bacterial detection which may cause interruption of bacterial growth.
Fluorescent methods have reported high sensitivity [8], but require fluorescent molecule labeling for sensing to occur. Labeling requires additional sample preparation and the fluorescent molecule can be degraded over long term exposure to liquid.
Electrochemical sensing can be used for selective detection a molecule without fluorescent labeling [9]. An electrochemical sensor array was integrated with a miniaturized bioreactor system for high throughput cell cultivation in 96 well plates [10]. Using a 100 μl working volume in the 96 well micro reactors, the sensor array can monitor temperature, pH, and oxygen concentration as well as total biomass. However, electrochemical sensors require a continuous power source for the operation and also require recalibration of the sensor due to the conductivity change of bacterial growth media in long term biofilm growth experiments.
Surface Acoustic Wave (SAW) sensors exhibit several advantages in small molecule detection including high sensitivity [11-22] and low power consumption [23]. A SAW sensor can detect mass or viscosity change due to the wave velocity attenuation, resulting in a resonant frequency shift at the output. A highly sensitive SAW sensor for detection of interleukin-6 (IL-6), which is one of key molecules in human immune system, was reported. In Krishnamoorthy et al. [19], a specific receptor for IL-6 was immobilized on the surface of the SAW sensor. Based on the resonant frequency shift due to the IL-6 binding, the detection limit of the SAW sensor was approximately 10−18 g (grams). A SAW sensor is also a passive device.
The power for operation of the SAW sensor can be delivered by an external device wirelessly which makes the SAW sensor useful for long term biofilm monitoring without a continuous power supply [23]. Furthermore, the SAW sensor can be fabricated using biocompatible materials [24-26]. The combination of extremely high sensitivity, biocompatibility, and low power consumption makes the SAW sensor a unique tool for real time monitoring of bacterial biofilm growth. However, it is also noted that piezoelectric materials used in the SAW sensor can be dissolved due to long term exposure to liquid [27].